This invention relates to phase lock of an output signal to an input signal and, more particularly, to a phase locking method and to a phase locking loop (PLL) circuit. In particular, the phase locking method is useful in an IC circuit. The phase locking loop circuit is convenient for implementation as an IC circuit.
In general, various phase locking loop circuits are known. An example is described in a tutorial paper contributed by Marc A. Rich to the IEEE Transactions on Communications, Volume COM-22, No. 7 (July 1974), pages 890 to 896, under the title of "Designing Phase-Locked Oscillators for Synchronization".
In the manner which will later be described in greater detail, a conventional phase locking loop IC circuit comprises a frequency and phase comparator responsive to circuit input and output signals for producing a frequency and phase difference signal representative of a frequency and phase difference between the circuit input and output signals. Connected to the frequency and phase comparator, a filter filters the frequency and phase difference signal into a filtered signal having a voltage dependent on the frequency and phase difference. Controlled by the filtered signal, a voltage controlled oscillator (VCO) generates a voltage controlled oscillation (VCO) signal which is phase locked for use as the circuit output signal to the circuit input signal and which has an oscillation frequency decided by the voltage of the filtered signal in an oscillation frequency range of the voltage controlled oscillator.
In IC circuits, it is usual that fluctuations are unavoidable in characteristics of elements in each IC circuit. The oscillation frequency range therefore differ from one phase locking loop IC circuit to another. As a consequence, it has been the practice to provide an adjusting terminal in the phase locking loop IC circuit to adjust the oscillation frequency range and a pull-in range on placing the phase locking loop IC circuit in a whole IC package.
Adjustment of the oscillation range is a superfluous step on completing the phase locking loop IC circuit. This results in a rise of cost of manufacture and in an undesirable increase of time in manufacturing a phase locking loop circuit.